Species, Units of Evolution, and Secondary Substance a Thesis

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Species, Units of Evolution, and Secondary Substance

A thesis presented to

the faculty of

the College of Arts and Sciences of Ohio University

In partial fulfillment

of the requirements for the degree

Master of Arts

Daniel J. Molter

June 2011

This thesis titled

Species, Units of Evolution, and Secondary Substance

DANIEL J. MOLTER

has been approved for

the Department of Philosophy

and the College of Arts and Sciences by

Arthur Zucker

Associate Professor of Philosophy

Benjamin M. Ogles

Dean, College of Arts and Sciences 3

ABSTRACT

MOLTER, DANIEL J., M.A., June 2011, Philosophy

Species, Units of Evolution, and Secondary Substance

Director ofThesis: Arthur Zucker

Species are classes of organisms on the traditional view, but David Hull argues that species as units of evolution are better understood as individuals composed of organisms as their parts. Phillip Kitcher counters that evolutionary species are better understood as sets. Following Elliot Sober, I argue that constituent definition prohibits a set-theoretic interpretation of species. Following John Dupré, I argue that species and units of evolution are ontologically distinct entities that require different names.

“Species” is the proper name of the species category, a class containing many intensionally-defined classes of organisms. The units of evolution that Hull describes are spatiotemporally-individuated physical objects. Following the principle of priority in biological nomenclature, Hull's transgenerational biological individual cannot be called

"Species", because that name has a prior valid use. I argue that Hull's species-as- individual has a prior valid name which can be found in Aristotle's Categories. Organisms are members of species, but they are parts of Secondary Substances.

Approved: ______

Arthur Zucker

Associate Professor of Philosophy

TABLE OF CONTENTS

Page

Abstract ...... 3 Chapter 1: Historical Introduction ...... 5 Chapter 2: Species as Individuals: A Critique of Hull ...... 10 Chapter 3: Why Species are not Sets ...... 46 Chapter 4: Species as Units of Classification ...... 54 Chapter 5: Units of Evolution as Substances ...... 63 Chapter 6: Postscript on Species and Units of Evolution...... 69 References ...... 70

CHAPTER 1: HISTORICAL INTRODUCTION

Until Darwin published The Origin of Species in 1859, scientific consensus held that species were unchanging natural kinds. Each organism was thought to share a common form characteristic of its species. An individual dog, for example, was thought to be an instantiation of the essence of dog in a particular quantity of matter. Being an instance of an essential form was taken as necessary and sufficient for membership in a species.

The notion of species as immutable natural kinds has its origins in ancient Greek philosophy. Plato held that plants and animals on earth derived their being from participation in perfect eternal forms. All pine trees, for example, were thought to be reflections of the perfect form of pine tree that exists in an otherworldly immaterial realm of forms. On the Platonic view, the species 'pine tree' is the collection of all individuals that derive their essence from participation in the perfect unchanging form of pine tree.

Aristotle also held that members of a species share a common essential form, but he rejected the Platonic notion of an immaterial realm of forms. The world itself is eternal on the Aristotelian view and the forms of plants and animals exist only in individual organisms. When plants and animals reproduce, their forms are copied in new matter, such that when an organism dies, its form does not perish but continues on, re- instantiated in the matter of its progeny. 6

Linnaeus inherited the Aristotelian notion of species as forms, but he modified the view to make it compatible with the Genesis story of creation. “We reckon the number of SPECIES as the number of different forms that were created in the beginning”

(Linnaeus 1751, 157). For Linnaeus, species were collections of animals and plants descended without modification from a single pair of created ancestors.

Linnaeus’s system of classification is a nested hierarchy of classes composed of five ranks: classes, orders, genera, species, and varieties (Linnaeus 1751, 155). The five ranks are based on the five categories of scholastic logical division: genus summum, genus intermedium, genus proximum, species, and individuum (Stafleu 1971, 28). Each rank in the hierarchy is related to the rank above it and to the rank below it by the genus-species relation. “The genus of the genera is the order, and the genus of the orders is the class” (Linnaeus 1751, 204). In this system of logical division, a genus is a general class of objects that possess common properties, and a species is a sub-division of the genus. Members of a species have all the properties required for membership in the genus, but they possess additional properties that differentiate them from other species in the genus. The genus-species relation is one of general sameness and specific difference.

Though their views on species differ in important ways, Plato, Aristotle, and

Linnaeus all held an essentialist species concept. That is, they all thought of species as collections of organisms sharing a common immutable form. On this view, the form of a thing is its essence, and it is the essence that determines what a thing is. Each thing can 7 have but a single essence, hence, every individual organism is a member of one and only one species. The goal of taxonomy under an essentialist species concept is to describe and catalog living things according to their essential forms (Popper 1950, Hull 1965,

Mayr 1969, Lehmann 1971; in Stafleu 1971, 25).

After Darwin's publication of The Origin of Species, the essentialist species concept became untenable. If one species can evolve into another, then the forms of living things do not remain constant from generation to generation. All essentialist accounts of species are refuted by the fact that gradual evolutionary processes cause the diversity of forms in nature. Essentialism was dead as a biological theory, but the concept of species did not fall with it. There are obviously specific kinds of plants and animals in nature, and that fact had to be explained in light of essentialism’s replacement by the theory of evolution.

In 1942, Ernst Mayr gave an account that he called the biological species concept.

"Species are groups of interbreeding natural populations that are reproductively isolated from other such groups." Mayr’s species concept avoids morphological essentialism by defining species in terms of relations between the members of a species. The forms of organisms in a species change over time in accordance with evolution, but the mating compatibility relation remains unchanged. Groups of organisms meet the conditions for membership in a biological species by maintaining reproductive compatibility in each generation of an evolving lineage. 8

Mayr's formulation captured the popular notion of what species are within the animal kingdom, but it had some weaknesses. It said nothing about species of asexual organisms that do not breed, and it was of little use for determining species of plants, which often form hybrids. Attempts to fill in the gaps left by the biological species concept resulted in the publication of more species concepts. By the late 20th century, over twenty different species concepts had appeared in journals of philosophy and biology (Mayden 1997).

Widespread acceptance of evolutionary theory ended the reign of essentialism as the dominant scientific theory, and it spawned a plethora of other a species concepts.

On the traditional view, all species concepts reduce to conditions for class membership.

Species concepts fall into three broad categories: phenetic, cohesive, and monophyletic

(Hey 2006). The three categories of species concepts are differentiated by the kinds of properties used as criteria for class membership. Phenetic species concepts compare the physical properties of organisms, cohesive species concepts consider relational properties, that is, how organisms relate to other organisms, and monophyletic species concepts rely on inferred properties, that is, organisms are segregated into species based on inferences about the history of their evolution.

In 1976, David Hull challenged the traditional notion of species as classes, claiming instead that species are individuals. In this master’s thesis I defend the traditional notion of species as classes. In the next chapter I examine Hull’s arguments for conceiving of species as individuals, and I argue that Hull has presented a mix of 9 good and bad arguments. In the third chapter I consider Philip Kitcher’s claim that species are sets rather than individuals, and I argue that species cannot be sets. In chapter four I outline John Dupré’s arguments from In Defense of Classification, and I agree with Dupré’s conclusion that species are classes. In the fifth chapter I argue that

Hull’s species-as-individual is actually a substance in Aristotle’s secondary sense. In the final chapter argue that a species can be defined theoretically as the class of all organisms that are parts of a secondary substance. 10

CHAPTER 2: SPECIES AS INDIVIDUALS: A CRITIQUE OF HULL

In Are species really individuals? David Hull argues that species are not classes.

On his view, evolutionary species are actually individuals. This chapter is a critical analysis of the arguments presented in that paper.

Whether species are individuals or classes or something else entirely, species are those things denoted by species names. This is common ground between classificationists and species-as-individuals theorists. All agree that the members of the species category are those things that are referred to by species names. Hull mentions three species names: Gorilla gorilla, Cygnus olor, and Drosophila melanogaster. The referents of these and similar species names are members of the species category1.

Hull’s paper has two related but distinct theses.

Thesis one (T1): Species are not classes.

Thesis two (T2): Species are individuals.

The conjunction of T1 and T2 serve to define the species category as a class with all and only individuals as its members.

1 Hull notes three distinct logical relations: class membership, class inclusion, and part-whole. The relation between particular species and the species category is that of class membership regardless of the ontological status of species.

“The relation between organisms, species, and the species category is membership. An organism is a member of its species and each species is a member of the species category. On the view being urged in this paper, both particular species and the species category must be moved down one category level. Organisms remain individuals, but they are no longer members of their species. Instead an organism is part of a more inclusive individual, its species” (Hull 1976, 174).

T1 and T2 are not mutually implied. T1 could be true and T2 false in case that species turn out to be something other than either classes or individuals. Alternately, T1 could be false and T2 true incase where species are thought to be both classes and individuals.

Hull must support both theses to accomplish the category shift he’s after.

Premise One (P1): Classes and Individuals are Ontologically Different. This Difference is Reflected in Language.

“The basic premise of this paper is that individuals are fundamentally different from classes (and other universals such as relations and processes) and that these differences must be reflected in language. The terms which denote individuals function differently from those that denote classes” (Hull 1976, 177).

Class terms are common names that refer to multiple individuals. Class terms have intensions and definitions–class terms are meaningful. Terms that refer to individuals are proper names. They lack intension and cannot be defined. “They are meaningless identification tags and nothing else” (Hull, 1976, 174). Individuals can be pointed out and they can be described, but they cannot be defined. When a description is precise enough to pick out one individual and exclude all others, Hull calls this a

“definite description”. Hull allows that some proper names are descriptive, but when this happens it is only accidental. The function of a proper name is to rigidly designate 12 an individual and nothing else. Individuals and proper names are linked up in a naming ceremony that Hull calls “Baptism”.

Intensional and Constituent Definition

“Two intensionally-defined class terms can denote exactly the same individuals and yet remain distinct. For example, if the names of taxa are viewed as class terms, then it just so happens that Mutica and Cetacea have the same known members. On the usual set-theoretic interpretation, however, classes are individuated on the basis of membership: two classes are identical if and only if they have the same members. Thus on this interpretation, Mutica is identical to Cetacea, a consequence no taxonomists could accept because Mutica is a cohort and Cetacea is an order (Gregg, 1954). However, if classes are interpreted intensionally, the two classes can be kept distinct (Buck and Hull, 1966). Two classes are identical if and only if the terms referring to them mean the same thing” (Hull 1976, 179).

In the Linnaean system, taxa names, including species names, are defined intensionally. The meaning of a class term is determined by a set of necessary and sufficient conditions for membership in the class. Sets on the other hand are defined solely in terms of their members. Sober calls this "constituent definition" (Sober 1980,

355).

According the Stanford Encyclopedia of Philosophy:

“The language of set theory is based on a single fundamental relation, called membership. We say that A is a member of B (in symbols A ∈ B), or that the set B contains A as its element. The understanding is that a set is determined by its elements; in other words, two sets are deemed equal if they have exactly the same elements” (Jech 2011).

The membership relation is operative both for an intensionally-defined class and for a set defined merely by the sum of its elements, but there is an important difference between classes and sets. Membership in an intensionally-defined class can wax and 13 wane without affecting the identity of the class. The principle of identity for the class is the meaning of its definition. Even if membership is reduced to zero, the class is preserved, because its principle of identity is its definition.

Sets identified by constituent definition lack this flexibility, which makes them unsuitable for most taxonomic purposes. The set of uranium atoms on August 5th 1945 is different from the set of uranium atoms on August 6th following the bombing of

Hiroshima. If the term “uranium” merely refers to a set of individuals rather than a kind, then the term’s referent shifts each time a uranium atom is created or destroyed.

Traditional taxonomy is the art of classifying things according to their kind. Kinds are intensionally-defined classes. Different sets of members populate an intensionally- defined class at different times, and this is non-problematic. The class remains constant because the set of conditions for membership in the class remains constant. When Hull claims that species are not classes, he means “class” in the sense of intensional definition.

There is at least one problem with Hull’s first premise. It’s not clear what difference there is between a definite description and the definition of a class with one member. If the definite description of an individual is really just a definition of a class with a single member, then the distinction between individuals and classes becomes a bit murkier than Hull would like it to be. “The first president of the United States” is a definite description that points to George Washington and only George Washington, but

“The first president of the United States” might also be taken as the condition for 14 membership in a class which contains the single member George Washington. Every definite description translates without loss into the definition of a class with a single member. Definite description appears to be a special case of class definition that occurs when a class has one and only one member. Identity between definite description and class definition is important for taxonomy if Hull is right and all the organisms in a species really are parts of a single individual. Hull could accomplish his goal of characterizing each species as an individual while retaining the logic of the genus- species relation which requires species to be classes.

Less problematic for the distinction between classes and individuals, but still curious, is Hull’s assertion that proper names are meaningless, a view he attributes to

Kripke. If species are individuals, as Hull argues, then the names of species are proper names. On this account Gygnus olor is a meaningless rigid designator wedded to a chunk of genealogical nexus by the taxonomist who named the species. The name serves only to designate an individual species and has no meaning beyond that.

Hull’s claim that species names are meaningless seems very odd given the rules that govern the naming of species. Cygnus olor is a Latin binomial assigned to a species in accord with a convention of naming developed by Linnaeus. Within that convention, species names are intended to be meaningful. The first part of the name tells us that

Cygnus olor is included in the genus Cygnus, a genus of birds in the family Anatidae. The binomial form of the name indicates that the name’s referent is a species as opposed to 15 a particular organism or a non-species taxon such as a family or an order. The species name tells us among other things that the referent of Cygnus olor is a species of bird.

When a person is christened any name will do, but when a species is christened its name must indicate the taxon’s rank as species, and the name must also indicate the genus under which the species is classified. If Zucker discovers a species of brilliant blue swans, it would not be appropriate to name the species Herbert, as that name does not indicate a taxon at the rank of species; nor would it be appropriate to name the species

Gorilla indigo, but either of these names would be acceptable if species names are meaningless rigid designators. Under the rules of biological nomenclature, a name like

Cygnus indigo would be more appropriate for a species of blue swan, regardless of whether species are individuals or classes. The colorful name carries semantic weight beyond rigidly designating its referent. Valid species names need not be descriptive in the way that Cygnus indigo is, but all species names are meaningful in that they indicate the genus to which the species belongs. Cygnus zuckeri is a less colorful name, yet we can still tell from the name that Zucker’s new species is a kind of swan.

Hull’s advocacy of Kripke’s curious account of proper names does not weaken his distinction between classes and individuals. Classes and individuals can still be ontologically distinct, and it is still possible that species names rigidly designate individuals in spite of their semantic richness. Hull would probably find the semantic weight of Latin binomials problematic, however, precisely because the names are defined under a class interpretation of species. The genus Cygnus is a class of birds, and 16 the species Cygnus olor is an included class, a class whose members differ in some way from all other birds in the genus Cygnus. Hull argues against the class interpretation, but it is a fact of convention that binomial species names find their meaning within the

Linnaean system. The descriptive claim, “Species names are meaningless” is false.

Maybe instead Hull is arguing that species names should be meaningless, but if the names were completely arbitrary and meaningless, then the order of any taxonomic system would fail to be reflected in a corresponding system of nomenclature – and that would defeat the purpose of taxonomy. Species names must include the genus under which the species is classified, and that makes the names meaningful regardless of whether species are classes or individuals.

Species names are meaningful by convention. Hull’s claim that they should be meaningless is not necessary for his larger project, nor is his argument in support of the claim convincing. Hull needs species names to be proper names if species are to be considered individuals, but he does not need proper names to be meaningless. Class names are defined by sets of conditions – they are necessarily meaningful, and their meanings are determined by the meanings of the words in their definitions. Terms that refer to individuals need not be meaningful but sometimes are. Proper names rigidly designate one and only one individual whether or not they have any other meaning. If species are individuals, as Hull argues, then species names are proper names and it does not matter that the names are also meaningful. 17

An example of meaningful rigid designation can be found in the “Bulls Eye” convention of naming compartments on navy ships. Each compartment is tagged in bioluminescent paint with a code that indicates deck number, frame number, and position relative to the centerline of the ship. Bulls Eye codes are like proper names.

Each code rigidly designates a spatiotemporally individuated compartment of the ship, yet each Bulls Eye is full of meaning. The Bulls Eye precisely indicates where a compartment is located relative to other compartments on the ship. Species names, even if species are individuals, serve a similar function. Binomial species names indicate where a species is located relative to other species in the genealogical nexus.

Premise Two (P2): Species are Units of Evolution.

Hull says that the theory of evolution is the primary theory of biological science, so the basic units of biological classification must correspond in some way to the basic units of evolutionary theory. Species should be identified with those things that evolve.

Hull acknowledges that some systematists disagree. On the contrary view, classification is a pre-theoretical exercise. Organisms must first be sorted into classes based on similar properties. Theoretical inference, evolutionary or otherwise, can be made only after the initial sorting is complete. Hull passes over this objection. Species must be identified with units of evolution by biologists “who believe that biological classifications must be in some sense evolutionary” (Hull 1976, 174). Classification is informed by theory, and the relevant theory is the Theory of Evolution. 18

Hull notes that Ernst Mayr and G. G. Simpson have identified species with units of evolution.

“Species are the real units of evolution, they are the entities which specialize, which become adapted, or which shift their adaptation” (Mayr 1969; in Hull 1976, 183).

“An evolutionary species is a lineage (an ancestral-descendant sequence of populations) evolving separately from others and with its own unitary evolutionary role and tendencies” (Simpson 1960; in Hull 1976, 184).

Argument from Type Specimens

The International Code of Botanical Nomenclature is a set of rules that governs the naming of plants and fungi. Similar codes regulate the naming of animals and bacteria. When a species is discovered and named, a specimen or a collection of specimens is preserved and placed in an herbarium to serve as a physical referent of the new name. As Hull notes (Hull 1976, 175), type specimens need not be typical. This is confirmed in article 7.2 of the ICBN.

“7.2. A nomenclatural type (typus) is that element to which the name of a taxon is permanently attached, whether as the correct name or as a synonym. The nomenclatural type is not necessarily the most typical or representative element of a taxon” (McNeill et al., 2006).

Species names refer to classes of organisms on the traditional view, but they refer to individuals on Hull’s view. Hull argues that the rules of biological nomenclature that wed names to type specimens are more consistent with a view of species as individuals than they are with the traditional view of species as classes. If type specimens are supposed to serve as exemplars of intensionally-defined classes, then one would expect the chosen specimen to exhibit most if not all of the properties that are severally necessary 19 and jointly sufficient for membership in the class. Properties mentioned in the definition should be evident in the type specimen. One would at least expect that the type specimen look like an average member of the species if species are classes, but the code does not require either of these.

If on the other hand species are individuals, then any organism that is part of a species can serve as the type specimen, even a diseased or abnormal organism. The parts of individuals need not be similar, and any part of the individual will serve to anchor the individual’s name. A blood sample or a hair sample can serve to link a person to a crime scene, because both are parts of the same individual. Analogously, any part of a species can serve to establish the referent of a species name. The code does not require that type specimens be typical or normal specimens. The rules governing selection of type specimens are more consistent with an interpretation of species names as denoting individuals than they are with an interpretation of species names as denoting classes. The argument from type specimens is a good argument in support of

T2.

Argument from Immutability of Classes

Hull’s central argument hinges on an incompatibility between the traditional view of species as classes and Darwin’s claim that species evolve. “If species are classes, it is difficult to see how they evolve—but they do!” (Hull 1976, 175). Darwin showed that species evolve. The traits of organisms in a species change over time through a process of mutation and natural selection. But if species are viewed as classes defined 20 by sets of properties that are “severally necessary and jointly sufficient”, then evolutionary change would not be possible. Organisms are included as members of a class precisely because they meet the definition of the class. If the properties of organisms in an evolving lineage change to the point where they no longer meet the requirements for class membership, then those organisms are no longer members of the class. If species are classes, and if species evolve, then the criteria for class membership would have to change in synch with the evolving traits of the organisms in the lineage, but classes don’t work that way. A class is identified by its definition, a set of necessary and sufficient conditions for membership in the class. If the definition changes then the principle of identity changes, resulting in a new class. Classes cannot evolve because the ontology of classes makes them necessarily immutable. If species evolve as Darwin convincingly argued, then species cannot be classes, because classes cannot evolve. This is a strong argument in support of T1.

Argument from Similarity to Organisms

The properties which make organisms individuals can also be found in species.

Hull proposes three essential properties for individuality: distinct position in space and time, continuity, and cohesion. Hull argues that species are cohesive, continuous, and spatiotemporally distinct to the same degree as organisms are. Particular organisms are the paradigm examples of individuals, so if species have the same individuating properties as organisms, then species are also individuals. 21

Spatiotemporal individuation means that each organism occupies a unique region of space and has a unique history. Two clones can be exactly similar in their inherent properties, but each occupies its own place in the universe and each has a different history. Two organisms with exactly similar properties do not make for a single organism, because each is spatiotemporally individuated. On evolutionary theory, species occupy localized regions of the genealogical nexus. Species exist in certain places and certain times. Species are individuated spatiotemporally just as particular organisms are. The argument from spatiotemporal individuation is a strong argument in support of

T2.

Individuals are continuous in both space and time. Spatial continuity is the property of organisms whereby all their parts are connected with no gaps between them. Temporal continuity is the property whereby an organism is the same individual in the morning as it is in the evening. The parts of an organism are connected in three dimensional space, and an organism remains the same individual as it develops though the fourth dimension which is time. Fourth-dimensional continuity or temporal continuity is especially important for the individuality of organisms that change radically from one stage of development to the next. A fly shares few properties with a maggot, but a fly is the same biological individual as the maggot it develops from. Similarly, a species can be viewed as the same individual from one historical period to another despite the accumulation of evolutionary change. 22

Species have continuity similar to organisms in that all organisms in a species are descended from a common ancestor, just as all cells in an organism are descended from a single zygote. Lines of genealogical descent tracing back to a common ancestor are lines of spatiotemporal continuity. There is no reproduction at a distance. At the beginning of its life, an organism is connected to its parent. The parent was likewise connected to its parents, and so forth. A strand of spatiotemporal continuity follows the chain of genealogical descent all the way back to the common ancestor of all organisms in the species. All organisms in a species are connected by interwoven lines of descent to form a continuous chunk of genealogical nexus. All the parts of the species are connected, so the species is a continuous individual.

The argument from continuity is not as strong as Hull might like it to be. All the cells in Gargantua’s body are descended from a single zygote. Analogously, all the gorillas in the species Gorilla gorilla are continuous by descent from a common ancestor. Continuity through descent is the same for organisms as it is for species, but organisms are continuous in a more obvious way. The parts of organisms are continuous in three-dimensional space, but the parts of species are discontinuous in 3D. Organisms are continuous in 3D and 4D, but species are continuous only in 4D. A snapshot of

Gargantua will show all his parts connected, but a snapshot of all gorillas will show that they are not connected. Organisms are continuous in every way that a species is continuous, but they are continuous in an additional way that species are not. It’s not clear what degree of continuity is required for membership in the individuals category. If 23 three-dimensional continuity is required for membership, then species are excluded from the individuals category. If 4D continuity is sufficient, then species might well be individuals.

Hull argues that some organisms lack 3D continuity. All organisms are individuals though, so it appears that 3D continuity is not required for membership in the individuals category. Slime molds live most of their lives as single cells moving independently, but when conditions are right many of these cells mass together into a

“slug” that moves as an individual. Slime molds lack 3D continuity during one stage of their lifecycle but are continuous three dimensionally during another stage. Organisms are individuals as the argument goes, so 3D continuity is not a necessary condition for being an individual.

One response to Hull’s slime mold example is to deny that slime molds are organisms. In fact many mycologists consider them to be colonies. The colony interpretation is common in mycology, not just for slime molds but for mushrooms too.

When a fungus grows to cover a substrate, a rotting log for example, the process is called colonization. On the colony interpretation of slime molds, each cell is an organism, and the aggregate ‘slug’ is a group of organisms working together, a migrating herd of single-celled organisms.

A more pointed response to Hull’s slime mold example is to note that a slime mold is three-dimensionally continuous at some point, but species never are. If slime molds lived their entire lives in the single-celled state, then Hull would have no reason 24 to consider a group of them to be an individual. It is only the fact that the individual cells sometimes come together that he has reason to consider the resulting “slug” an organism. 3D continuity is the property sometimes possessed by slime molds that prompts Hull to call them individuals. 3D continuity is the property sometimes lacking in slime molds that makes then interesting borderline cases for individuality. Species like

Gorilla gorilla perpetually lack 3D continuity. In this respect, species are not continuous even to the extent that slime molds are. Even if one considers slime molds to be organisms, the stark contrast in continuity between organisms and species remains.

Every organism is continuous in 3D at some point in its development. Species remain discontinuous in 3D throughout their evolution, so it is not the case that species are just as continuous as organisms are.

Cohesion is what holds the parts of an individual together. For all the parts of an organism to remain continuous something has to hold them together. Bones in a skeleton are held together by ligaments for example. Cohesion in a three-dimensionally discontinuous individual like a species is more nuanced. Hull notes that continuity by descent is not enough to make a lineage into a unit of evolution. All life is descended from a common ancestor on the standard view, so every organism has continuity by descent with every other organism in the genealogical nexus. Cohesive factors shared by organisms in a lineage make the lineage a unit of evolution distinct from other evolving lines. On Hull view, organisms are parts of a species, and those parts must be organized in such a way that the species can function as a unit of evolution. 25

Interbreeding is the inherent cohesive element in sexual species. Individual lines of descent within a species are woven into complex reticulate networks, analogous to the way single hairs are woven into a braid. Reproductive isolation from other species marks the outer edge of the braid formed by interwoven lines of descent. Reproductive isolation also marks the ‘edge’ of the gene pool for a species. Mating compatibility and reproductive isolation provide the strong glue needed to hold a three-dimensionally discontinuous group of organisms together as a single unit of evolution.

Cohesion is problematic for asexual species. “Asexual species lack any intrinsic mechanism for promoting evolutionary unity. If asexual organisms form cohesive units of evolution, they must do so entirely on the basis of the unifying effects of external causes, and considerable doubt exists with respect to the nature and efficacy of such external causes” (Hull, 1976, 184). Ernst Mayr and Michael Ghiselin held that asexual organisms do not form species (Hull, 1976, 183), but Hull notes that Simpson leaves open the possibility in his formulation of evolutionary species. All agree that asexual lineages can and do evolve. Hull has to bite a bullet either way. He can follow Mayr and

Ghiselin in supposing that there are no asexual species, but in doing so he must abandon the notion of species as units of biodiversity. On the other hand, Hull can affirm that there are asexual species but their cohesion is suspect and they might not be units of evolution. Hull punts; “Perhaps groups of sexual and asexual organisms do not form the same kinds of evolutionary units, perhaps they evolve in strikingly different 26 ways, perhaps their evolution is governed by distinct sets of laws, but they nevertheless evolve” (Hull 1976, 184).

Sexual reproduction weaves many individual threads of descent into a common fabric that maps the genealogical history of a species. Individual lines of descent in the genealogical matrix of a sexual species both bifurcate and merge. Every sexual cross binds two lines into one. All lines of descent in a sexual species frequently cross and bind with other lines of descent; this holds the species together as a unit of evolution. It is not important that organisms in a species be similar morphologically; “cohesion” is not a metaphor for morphological similarity. Crisscrossing lines of descent make the species into a tightly woven chunk of genealogical nexus that is spatiotemporally discontinuous with other similarly organized chunks.

Asexual species have posed a problem for species theorists since Mayr proposed the biological species concept, so it’s not surprising that they are also problematic for

Hull’s account of species as individuals. Organisms in an asexual species are all descended from a common ancestor, but particular lines of descent never cross. Lines of descent in asexual species bifurcate but they never merge. Compared to the woven fabric of a sexual species, the genealogy of an asexual species resembles a perpetually branching tree. It’s not clear what could possibly bind the branches together into a unified whole. Much talk about ecological factors being cohesive is really just a metaphor for morphological similarity, but similarity does not make for individuality. A similar niche provides similar selection pressures on multiple linages, leading to 27 similarity in form of the organisms constituting the different lineages, but this is more like a special case of convergent evolution than it is like cohesion in a sexual species.

Organisms in multiple asexual lineages can be morphologically and ecologically similar, but the lines of descent are not woven into a whole. For this reason, asexual species also lack a common pool of genetic resources. Individual lines of descent in an asexual species are like pipes for genetic flow; the pipes do not empty into a common pool. If there are factors that bind disparate asexual lineages into a cohesive individual it’s not clear what those factors are. Asexual species do evolve though, so perhaps the units of evolution need not be individuals.

Hull argues that species and organisms share three properties that make both individuals: spatiotemporal distinctness, continuity, and cohesion. With respect to spatiotemporal distinctness, the comparison is symmetrical. Like each organism, each species blazes its own unique trail through space and time. With respect to continuity, species are continuous to a lesser degree than organisms are because they lack three- dimensional continuity. With respect to cohesion, the lines of descent in sexual species frequently cross forming the species into a tightly woven chunk of genealogical nexus.

Asexual species lack this cohesive factor, and Hull is not clear about what if anything could serve to mold the branching lines of an asexual species into a cohesive unit of evolution. What is clear is that all species share one property of individuality with organisms and that all species share a second property of individuality with organisms, though to a lesser extent. Some species share the third criterion for individuality but 28 other species do not seem to share the third property with organisms. Everyone agrees that organisms are individuals. Whether or not species are also individuals depends upon the criteria for membership in the individuals category. Species share some of the individuating properties of organisms but lack others. Hull’s arguments from comparison to organisms supports T2, but the support is not as strong as Hull would like it to be.

Argument from Dissimilarity to Chemical Elements

Hull contrasts species of living things with kinds of chemical elements to show how they are different.

Lack of Essence: Gold is the class of all atoms with atomic number 79. Having 79 protons is both sufficient and necessary for membership in the class of things that are gold atoms. It is not accidental that each atom in a gold bar contains 79 protons. Atomic number 79 is the essence of gold. All of gold’s properties – malleability, yellow color, melting point, etc. – follow from gold’s atomic number and the laws of subatomic physics. Hull argues that organisms in a species lack such an essence. Genetic code serves as an essence for individual organisms but not for species. The genetic code of an organism is analogous to the atomic number of a chemical atom in that properties of an organism follow from its genetic code in the same way that the properties of gold follow from its atomic number. Living things differ from chemical elements though in that each has a unique genetic code. All gold atoms have exactly 79 protons, but each gorilla has a slightly different genetic code. 29

When intensionally-defined, gold is the class of all atoms that meet a set of conditions; the set contains a single condition, having atomic number 79. Attempts to define Gorilla gorilla in a similar manner to gold yields a polythetic set of conditions for class membership, and as a result, gorilla becomes a cluster concept (Hull, 1965). A polythetic or disjunctive set of conditions for class membership is not the same as an essence. “Essentialism is the view that all genuine classes have essences” (Hull 1976,

176). Gold has an essence, so gold is a genuine class. Gorilla gorilla lacks an essence, so, on Hull’s view, it is not a genuine class. I find this argument to be rather weak.

Polythetically defined classes are still classes. A polythetically defined species might not be the same kind of class as the class of all gold atoms, but this does not make it a non- genuine class. The class of logical operators, for example, is defined polythetically, but it is nonetheless a genuine class.

Commonality of Origin: Gold atoms can be created in a number of different ways at a variety of places in the universe. Most gold is made at the center of a collapsing star from the fusion of lighter elements, but some gold is made in nuclear reactors from the fission of mercury. The origin and cause of an atom is not important for determining whether or not the atom is gold. If the atom has 79 protons, then the atom is gold; its origin is not relevant to its essence. Species of living things are different. To be a gorilla is to be born of a gorilla. If on another planet astrobiologists were to discover creatures that had all the inherent properties of gorillas, then those creatures would not be parts 30 of the species Gorilla gorilla, because they are not descended from the common ancestor of terrestrial gorillas.

Permanence of Extinction: If all gold atoms in the universe were annihilated, a slot for gold would remain open on the periodic table. Gold is an intensionally-defined class whose principle of identity is a set of conditions for class membership. Even if the membership is reduced to zero, the conditions for membership remain untouched. If at some point after the annihilation a new atom meeting the conditions is created, then gold would reemerge to fill the open slot on the periodic table. If gorillas go extinct, there is no chance that they will ever evolve again. If a new species of animal were to evolve that looked like gorillas and acted like gorillas, those animals would fail to be gorillas; they would constitute a new species with a unique place in the genealogical nexus.

The arguments from dissimilarity to chemical elements support both T1 and T2, but in a rather circular way. One must accept Hull’s notion of species as historical individuals for the arguments to work. A recalcitrant classificationist could propose contraries to all of Hull’s claims. One might say that species do have essences; we just haven’t found them yet. If animals that resemble gorillas in every way are discovered on another the planet, then those animals could be considered gorillas regardless of origin -

- and evolutionary theory would have to be modified to account for them. If gorillas become extinct and then a new line of animals evolves that are like gorillas in every way, then the new animals might well be considered gorillas, and we would have to 31 tweak evolutionary theory a bit. Despite the apparent circularity, the argument from dissimilarity to chemical elements is still a good argument. It seems very unlikely that indistinguishable organisms could ever evolve from two different lines of ancestry. The odds of finding animals that are indistinguishable from gorillas but not descendant from gorillas are so remote that even the recalcitrant classificationist would likely agree with

Hull that species of living things are in fact, if not in principle, historically restricted.

Argument from Increased Coherence of Evolutionary Theory

“The basic unit of classification must be some basic unit of evolution” (Hull,

1976, 174). The units of evolution are those things that evolve. Because classes cannot evolve, the units of evolution cannot be classes. If species are to be identified with units of evolution, then a species must be of an ontological type that can change its properties while maintaining its principle of identity. Individuals meet this requirement, so Hull argues that species are actually individuals.

Hull claims that the ontological status of species, whether they are classes or individuals, must be determined by the theory in which the term “species” is used. “The choice between these two interpretations cannot be made on the basis of simple empirical considerations but on the basis of the increased coherence permitted by one interpretation over the other” (Hull 1976, 175). Species are the units of diversity in biological science, and evolutionary theory is the primary theory of biology, so if evolutionary theory is to be made more coherent by interpreting species as individuals, then it follows that we should interpret species as being individuals rather than classes. 32

All of Hull’s arguments are directed toward showing that evolutionary theory is more coherent when species are considered as individuals.

According to Hull, evolutionary theory describes the interplay of three natural phenomena: mutation, selection, and evolution. Mutation is a change in form from parent to offspring usually caused by a reordering of genetic code. Selection is the process by which some organisms survive to reproduce while others do not. Evolution is the process whereby the traits of organisms in an ancestor-descendant lineage change, usually over many generations. Each of these natural processes is associated with a group of entities that undergo the processes. Units of mutation are individuals that mutate, units of selection are individuals that are selected for or against, and units of evolution are individuals that evolve. “Dogmatically, the gene is the unit of mutation, the organism is the unit of selection, and species is the unit of evolution” (Hull 1976,

181). Hull acknowledges that some biological individuals function in more than one role.

Genes might be units of selection as well as units of mutation. Populations might serve as both units of evolution and units of selection. Key to Hull’s argument is that all three components of evolutionary theory – units of mutation, units of selection, and units of evolution – are individuals. Evolution might occur at the level of kinship groups and populations as well as at the level of species, but wherever evolution occurs, there is some individual that is evolving. When the evolving thing is a species, the species must be an individual. Because evolutionary theory describes the interplay of individuals,

(individuals mutating, individuals being selected, and individuals evolving) evolutionary 33 theory is more coherent when all interacting parts, species included, are conceived of as individuals. This argument supports T2, but it has some weaknesses.

Hull notes that most biologists consider the process of evolution to be more complex than the dogmatic account where organisms are the units of selection. “As

Lewontin (1970) has argued, selection occurs at an even wider range of levels of organization, from macromolecules to kinship groups, probably at the level of populations, possibly even at the level of species” (Hull 1976, 182). It seems evident to me that selection frequently occurs at the level of species. Early human hunters and changing global climate selected the wooly mammoths right out of existence. The mammoths were not fit enough to survive a conjunction of diminished habitat and increased predation, so the species went extinct. Natural selection is a process that drives extinction as well as evolution. Whenever there is an extinction event, there also is an instance of a species acting as a unit of selection.

Hull claims that units of selection are always individuals, regardless of the level of biological organization at which they occur. “Entities at various levels of organization can function as units of selection if they possess the sort of organization most clearly exhibited by organisms; and such units of selection are individuals, not classes” (Hull

1976, 182). I disagree with Hull on this point. In some cases it appears that nature selects classes of organisms rather than individuals. Coral reefs are bleaching all over the world right now because ocean temperatures are rising. Corals that cannot tolerate the increased temperatures are being selected against while those that can take the 34 heat are being selected for. This process is happening without regard to the species of corals involved. Some individuals in each species are perishing due to increased temperatures while other conspecific individuals manage to survive. A broadly polyphyletic class of coral appears to be functioning as a unit of selection. Granted individual corals are being selected, but selection pressure is on a class of organisms sharing a common property; the class does not constitute an individual. All corals with the property of heat intolerance are dying, and as such the unit of selection is a class defined by the property of heat intolerance. Selection pressures that targeted the wooly mammoths were likewise not directed at a particular lineage so much as they were directed at a class of animals that shared common properties. Other species of large animals went extinct at the same time the wooly mammoths vanished for precisely the same reasons. Large size was the property that reduced fitness in a time of habitat loss and increased predation by groups of organized weapon-bearing hunters.

It’s difficult to think of a class functioning as a unit of evolution, but the extinction of wooly mammoths shows that species can function as units of selection, and the bleaching of corals shows that sometimes units of selection are most coherently interpreted as classes. These examples expose a weakness in Hull’s account of evolution as a process that requires all the interacting parts to be individuals.

Argument from Integrated Levels of Material Organization

“Nothing is more obvious about the living world than the existence of intermeshed levels of organization from macromolecules, organelles, and cells to organs, organisms, and kinship groups. Each of these levels is related to the one above it by the part-whole relation, not class membership or class inclusion. The main concern of this paper is whether a radical break occurs above the level of individual organisms and/or kinship groups. Are organelles part of cells, cells part of organs, organs part of organisms, and possibly organisms part of kinship groups, but organisms are members of populations and/or species? I think not. The relation that an organ has to an organism is the same as the relation which an organism has to its species” (Hull 1976, 181).

When considered as a standalone argument, Hull’s argument from integrated levels of material organization is dogged by spatial discontinuity at levels of material organization above that of individual organisms. The radical break in material organization that Hull questions is the break between 3D continuity and 4D continuity.

Hull says that organisms are “possibly” parts of kinship groups, but possibly they are not parts of kinship groups. The levels of material organization from molecule up to organism intergrade within a three dimensional substrate that is the body of an organism. Macromolecules, organelles, and organs are all parts of the same three- dimensionally continuous individual. The part-whole relation between an organism and a kinship group differs in that the kinship group is not continuous in 3D. The part-whole relation might still be applicable, but only in the sense that three-dimensionally discontinuous groups of objects can be considered wholes.

In light of Hull’s earlier arguments that species are spatiotemporally-individuated chunks of genealogical nexus, objections from 3D discontinuity lose much of their force. 36

The part-whole relation is transitive. If a cell is part of a liver, and the liver is part of

Gargantua, then the cell is part of Gargantua. On Hull’s view, the liver cell is not only part of Gargantua, it is also part of Gargantua’s kinship group, his population, and the species Gorilla gorilla. On the view that species are continuous chunks of genealogical nexus, part-whole language better describes the relation between an organism and its kinship group, because it allows for parts of organisms to also be parts of the larger historical individual. If on the contrary view organisms are considered to be members of their kinship group rather than parts, then transitivity no longer holds; a liver cell is not a member of a kinship group because only organisms are members of kinship groups.

When genealogical lineages are viewed as continuous physical objects, parts of the lineage’s parts are also parts of the lineage. When considered in conjunction with Hull’s arguments for spatiotemporal individuation and continuity of species, the argument from integrated levels of material organization supports both T1 and T2.

Argument from Non-continuity of Classes

On Hull’s view, classes are universal, and therefore not restricted to certain places and times. Species are “chunks of genealogical nexus”. They are historical entities restricted to particular regions of space and time. Chunks of genealogical nexus are composed of organisms connected by ancestor descendant relations. The ancestor descendant relation entails not only spatiotemporal restriction, but also spatiotemporal continuity. In the closing sentence of Are species really individuals?, Hull claims that spatiotemporal continuity is the property of species that prevents them from being 37 classes. “Whether or not spatiotemporal continuity is also necessary for something to be an individual, it is sufficient for not being a class” (Hull 1976, 190). Interestingly, Hull revises this claim in a later paper when he allows that the members of some classes2 can be localized, but he says this renders the classes non-genuine.

“One might distinguish two sorts of sets: those that are defined in terms of a spatiotemporal relation to a spatiotemporal localized focus and those that are not. On this view, historical entities such as Gargantua become sets. But they are sets of a very special kind—sets defined in terms of a spatiotemporal relation to a spatiotemporally localized focus. Gargantua, for instance, would be the set of all cells descended from the zygote which gave rise to Gargantua… the reason for distinguishing between historical entities and genuine classes is the differing roles which each plays in science according to traditional analyses of scientific laws” (Hull 1978, 337).

Organisms in a species are descended from a common ancestor in the same way that cells in particular organisms are descended from a single zygote. We can define species as classes in terms of relation to a common ancestor, but the same reasoning allows us to define particular organisms as classes of cells. Organisms are the paradigm example of individuals, so the desire is not to conceive of organisms as classes.

Hull presents two arguments here in support of T1, an ontological argument and a pragmatic argument. First we are told that species cannot be classes because spatiotemporal continuity is antithetical to the nature of classes. Later Hull allows that

2 Hull uses ‘set’ and ‘class’ interchangeably in the following quote, which looks like a mistake given his previous distinction between intensional and constituent definition. Sets can be translated into classes with disjunctive sets of conditions for membership, but proper classes do not translate into sets. The set which contains the elements Dan Molter, the number five, and Lactarius indigo, for example, can be rewritten as the class of all things that are either Dan Molter, or the number five, or Lactarius indigo. One may not however translate an intensionally-defined class into a set defined by its constituents – as illustrated by the uranium example above. Hull’s intent regarding the group of cells descended from the zygote that developed into Gargantua seems more consistent with intensional definition than with constituent definition. He probably should have used “class” in place of “set” in the following quote.

38 classes can have spatiotemporally restricted members but that it is better to interpret these kinds of classes as individuals. Hull’s first argument concerns the ontology of classes, but his later argument is pragmatic; Hull thinks that science is better served when both species and organisms are interpreted as individuals. In response to the pragmatic argument, it seems quite obvious that in some situations the individuals interpretation of species better serves science while at other times a class interpretation is more useful. An evolutionary theorist attempting to construct of phylogeny of primates would be well served by considering Gorilla gorilla to be an individual. An ecologist attempting to estimate the number of gorillas in a nature preserve would be better served by considering the species as a class of animals.

Both arguments from spatiotemporal restriction fail to support T1. The first argument looks like a category mistake; it conflates a class with the members of the class. Plato noted a metaphysical line which divides abstract reality from physical reality.

Classes fall on one side of the line and individuals fall on the other. Classes don’t exist anywhere, so it is not possible for a class to be spatiotemporally restricted, though a class’s membership can be. Classes are universals akin to justice and perfect circles.

They are ideas that can be defined, but they cannot be confined by space and time. The members of a class on the other hand, if the members are physical objects, do exist in space and time; all physical objects are spatiotemporally restricted. It is quite possible for all the members of a class to be located in close proximity to each other and yet the class remains a universal. A non-biological example will help to illustrate this. 39

Consider the class of all objects that are Carson City silver dollars. Carson City silver dollars are distinguished from other US silver dollars by the mint mark “CC” which appears under the word “dollar” on the face of the coin. All Carson City silver dollars are chunks of metal mined from the Comstock Lode, a large silver deposit near Carson City

Nevada. At the Carson City mint each chunk of metal received its distinctive markings.

The Class ‘Carson City Silver Dollar’ is defined by a set of properties that all its members share. Each has a certain shape and weight, material composition, and history of production. Anywhere in the universe where these traits coincide in a single coin, there exists a Carson City silver dollar. The fact that all Carson City silver dollars happen to exist on earth does not render the class definition non-universal. The members of all classes of physical objects exist in localized regions of space and time, yet the classes remain universal.

Carson City silver dollars are both restricted and continuous in space and time.

The material for each coin originated from the same silver deposit, and each coin traces its history back to the Carson City mint. Each coin has a common ancestor in the

Comstock Lode, and each coin has of history of being stamped at the Carson City mint.

Common ancestry and common history make for spatiotemporal continuity, a property that Hull says is incompatible with being a class, yet it seems very strange to consider a group of coins dispersed throughout the world as an individual. Here again, the class is universal, even though its members are restricted and continuous in four dimensions.

Even if all the coins are parts of a larger individual due to spatiotemporal continuity, a 40 dubious claim considering the lack of cohesive factors, Carson City silver dollars are nonetheless members of a universal class that can be defined. This example shows that a class can remain universal even in cases where all the class’s members occupy a particular continuous region of space and time.

Hull’s ontological claim, that spatiotemporal continuity is sufficient to prevent something from being a class, is true only in the trivial sense that classes as abstractions are non-spatiotemporal entities; no class whatsoever can be continuous or restricted in space and time. If on the other hand Hull’s claim concerns the membership of a class, then the ontological claim is false. As the Carson City silver dollar example shows, it is quite possible for a class’s membership to be spatiotemporally restricted and continuous.

Hull’s pragmatic claim is also suspect. In some situations it might be better to think of species as individuals, but in other situations the class interpretation is more useful. Classes have definitions which serve as the principle of identity for the class.

When species are conceived of as classes, the definition of the species also serves as the principle of identification for the specimen. It may well be true that all gorillas are parts of a single transgenerational individual, but this ‘fact’ of evolutionary theory does not help the field biologist determine whether the large hairy ape up in that tree is a gorilla rather than a chimpanzee or some other kind of animal. In order to make that determination, she must observe the animal’s properties and compare those properties to the conditions which must be met to properly identify an animal as a gorilla. 41

Continuity with a known unit of evolution can serve as the principle of identification for a specimen only in cases where that continuity is observed. A microbiologist who inoculates a petri dish with a single bacterium and subsequently watches it grow into a colony can infer, provided there is no contamination, that all bacteria in the colony are parts of the same continuous chunk of genealogical nexus. In cases where continuity is observed, continuity can serve as the principle of identification, but outside the lab continuity is rarely observed. In cases where continuity is not observed, biologists employing an evolutionary species concept must infer continuity with a unit of evolution from properties observed in the specimen. In such cases a class interpretation of species is required. When the goal of science is to identify specimens, the class interpretation of species with its conditions for species membership is the only workable option. Hull’s pragmatic claim that science functions better when species are interpreted as individuals is only partly true. Sometimes the class interpretation of species is the only workable interpretation. Both of Hull’s arguments from non-continuity of classes fail to support T1. The ontological argument fails because classes can and do have spatiotemporally restricted and continuous members. The pragmatic argument fails because some scientific endeavors such as identifying specimens require a class interpretation of species.

Argument from Spatiotemporal Restriction and Universality of Scientific Laws

Hulls offers this argument in defense of premise one against the possibility that classes and individuals need not be distinct, that a species might be both a class and an individual.

“The most important reason for retaining the class-individual distinction is the differing roles each plays in science. On the usual analysis, scientific law is supposed to characterize timeless regularity in nature. It must be spatiotemporally unrestricted. To the extent that a law of nature is true, it must be true anywhere and at any time” (Hull, 1976, 187).

The speed limit in Ohio is 65 miles per hour, but the speed limit in West Virginia is 70 miles per hour. Speed limits are not scientific laws, among other reasons, because they are restricted to certain places and times. Scientific laws range over classes of objects that share common properties wherever those objects appear in the universe.

Generalizations about the members of a class defined by relation to a spatiotemporally located individual lack universal jurisdiction, so these generalizations can’t be scientific laws. On Hull’s view, statements such as “Water boils at 100 degrees at sea level” and

“Dogs breath oxygen” fail to be scientific laws. These statements appear to be both scientific and law-like, but the first statement explicitly references a location in space, and the second statement ranges over a class of animals descended from a spatiotemporally-localized common ancestor. Both claims hold true only on Earth, so on

Hull’s view both statements would fail to be scientific laws. 43

Hull’s argument, if sound, does more than prevent a class interpretation of species. If the argument from universality of scientific law works to prevent species from being construed as classes, then it also prevents a class interpretation of higher taxa. All life shares a common ancestor on the received view, so no scientific laws could be discovered that range over all and only flowering plants or all and only eukaryotes. Biota in its entirety must be viewed as an individual composed of parts descended from a common ancestor rather than the class of all living things. If Hull is right, then no scientific laws can range over all organisms in the genealogical nexus, nor can there be scientific laws that range over groups of organisms in any sector of the nexus. There can be no scientific laws that range over taxonomic categories at any rank. By Hull’s reasoning, eukaryotes should not be construed as a class of organisms characterized by the presence of a nucleus, because “Eukaryotes have a nucleus” is not a scientific law.

This reasoning appears to be flawed. It seems to me that science still has good reasons to consider eukaryotes as a class of organisms distinct from prokaryotes even if Hull is right and scientifically significant generalizations about eukaryotes don’t count as scientific laws. Hull’s argument also entails that no scientific laws can range over parts of organisms, which are spatiotemporally-restricted by the same mechanism that organisms are. “Red blood cells carry oxygen” cannot be a scientific law, because all red blood cells are descended from a common ancestor and all red blood cells exist in animals on earth. Statements about the composition and function of DNA would also fail to be scientific laws on Hull’s analysis of scientific law, because all DNA yet discovered is 44 descended from a common ancestor. Even so, the class of red blood cells and the class of DNA molecules are still scientifically useful categories. If Hull is right, then none of the law-like generalizations of biology count as scientific laws, but is does not follow that these generalizations should be abandoned.

Those who think of biology as a science governed by laws might object to Hull’s views on scientific laws, but I will not challenge him on that front. For the sake of argument I will assume that there cannot be scientific laws that range over particular species. However, it does not follow from this assumption that species shouldn’t be construed as classes. Biologists make scientifically significant generalizations about the members of species, and these generalizations require a class interpretation of species.

It might be the case that the scientifically significant generalizations of biology are not scientific laws, but these generalizations are required for biology to function as a science, and generalizations about organisms in a particular species are possible only when species are construed as classes.

Conclusion

Hull offers some good arguments in favor of both T1 and T2, but some of his arguments fail to support either thesis. If we accept premise 2, that species are units of evolution, then the argument from immutability of classes is a strong argument that species are not classes. The arguments from similarity to organisms and dissimilarity to chemical elements support the concept of species as individuals, though not as strongly as Hull would like. Three-dimensionally continuous objects differ significantly from 45 objects which are continuous in the fourth dimension but lack 3D continuity. It is not clear which degree of continuity is required for individuality. The argument from increased coherence of evolutionary theory is weakened by the fact that units of selection can be classes and species can be units of selection. The arguments from spatiotemporal restriction and universality of scientific laws fail to support T1.

Scientifically significant generalizations about organisms in a species require a class interpretation of species. Generalizations are required for biology to function as a science, even if those generalizations are not proper scientific laws.

All of Hull’s arguments, except for possibly the argument from type specimens, turn on an identity between species and units of evolution. In the fourth chapter I challenge that identity relation.

CHAPTER 3: WHY SPECIES ARE NOT SETS

Philip Kitcher argues against Hull’s claim that species are individuals. Kitcher says that on the traditional view, species are sets of organisms. This part of the traditional view does not entail essentialism.

“There is no inconsistency in claiming that species are sets and denying that the members of these sets share a common property. Unless “property” is used in an attenuated sense, so that all sets are sets whose members share one trivial property—namely, the property of belonging to that set” (Kitcher 1984, 310).

On the standard interpretation of set theory mentioned in the previous chapter,

Kitcher’s assertion is true. Sets are not defined intensionally in terms of conditions which must be met for membership. The principle of identity for a set is merely its membership. The members of a set need not share any trait beyond being members of the set.

Kitcher describes an evolutionary species as a set of organisms descended from a common ancestor.

“Assume, for the sake of the present argument, that a species is a set of organisms consisting of a founder population and some (but not necessarily all) of the descendants of that population. I make this assumption to show that there is a set theoretic equivalent to the approach to species that Hull favors. For any given time, let the stage of the species at that time be the set of organisms belonging to the species which are alive at that time. To say that the species evolves is to say that the frequency distribution of properties (genetic or genetic plus phenotypic) changes from stage to stage. To say that the species gives rise to a number of descendant species is to claim that the founding populations of those descendant species consist of organisms descending from the founding population of the original species. By proceeding in this way, it is relatively easy to reconstruct the standard claims about the evolutionary behavior of species (Kitcher 1984, 311). 47

Hull’s account of species as individuals and Kitcher’s account of species as sets are very similar. In both cases organisms in a species are connected to each other through lines of descent, and on both interpretations the properties of organisms in a species differ from one historical period to another. On Hull’s view all the organisms in a species are parts of a larger historical individual, but on Kitcher’s view the organisms in a species are members of a set, where some subsets define historical stages.

Both views require that dead organisms be included in the species. For Hull, dead organisms are parts of the historical individual. Three-dimensionally discontinuous groups of organisms find 4D continuity through connections to ancestral organisms that have long since perished. For Kitcher, dead organisms are members of subsets corresponding to temporal stages. The set of living gorillas in 1900 shares no members with the set of living gorillas in 2010, but both sets of gorillas, the living and the dead, are members of the trans-temporal set Gorilla gorilla. It’s not clear how the part-whole relation and the set-member relation are distinct here. The members of the set map perfectly onto the parts of the whole, and this seems to be Kitcher’s point. If the set that

Kitcher describes is logically indistinguishable from the individual that Hull describes, then it appears that sets can evolve.

Problematic for Kitcher’s account is that a set may not take on new members while at the same time retaining its identity. An individual can grow a new part, but a set is eternally wedded to its membership by constituent definition. The set of all gorillas at time T1 is defined as a certain number objects that are or ever have been 48 gorillas. At T2 after a new gorilla is born, the set of all objects that are or ever have been gorillas is constituted by different members, making it a different set than the set at T1 before the new gorilla was born. The sets at T1 and T2 have different constituents, and therefore different principles of identity; they are two different sets. What Kitcher describes appears not to be a set, but rather a series of sets.

Kitcher’s set-theoretic interpretation of an evolutionary species works from a backward-looking perspective, but it does not work when considering the future evolution of a species. Looking back we see a series of connected temporal stages; each stage is a subset of the larger set. When considering future membership in the species, however, the set-theoretic interpretation breaks down; sets are defined by their membership, and I see no good way to accommodate an unknowable quantity of future elements in a set defined by its membership. Kitcher might be able to save a set- theoretic account of species by including potential organisms in the set of elements that constitute a species. The set Gorilla gorilla would then contain existing gorillas, formerly extant gorillas, and potentially existing gorillas. Such an account would be problematic though, because there is no way to determine how many potential animals are members of the set. One might also question the reality of potential gorillas, and the validity or usefulness of a set that contains a mix of actual and potential members.

Potential membership is unproblematic on the class interpretation of species.

Classes are defined by sets of conditions. The conditions for membership do not change, and whenever those conditions are met a member is added to the class. Hull’s species- 49 as-individuals can also accommodate potential organisms. A tree does not become a different individual when it grows a new leaf; neither does the species Gorilla gorilla become a different individual when a new gorilla becomes a part of the species-as- individual.

On Hull’s account, units of evolution must be continuous and cohesive, but they must also be capable of “open ended development” (Hull 1976, 182). Organisms can’t be units of evolution because death closes off their potential for future development.

Sets lack open-ended development for a different reason; sets can’t develop at all. A class cannot evolve because the conditions for class membership may not change, but a set cannot evolve because the number of its members may not change. Evolution requires both new organisms and new properties for those organisms, so units of evolution can be neither sets nor classes; sets have immutable membership, and classes have immutable definitions. Individuals on the other hand can persist despite changes in both constitution and character; historical individuals can have both different constituents and different properties at different points in time, so individuals appear to be the best candidates for units of evolution.

Argument that Some Species Might not be Continuous

Kitcher cites the Checkered Whiptail Lizard, Cnemidophorus tesselatus, as a species which might not be continuous. If a species can be non-continuous, then species are not individuals. Kitcher describes C. tesselatus as a parthenogenetic3 species that evolved from hybridization between two bisexual species. According to Kitcher,

“Cnemidphorus tesselatus has resulted from a cross between C. tigris and C. septemvittatus (Parker and Selander 1976, Parker 1979)” (Kitcher 1984, 314). The hybrid is morphologically and genetically distinct from the parental species, and its unisexual reproductive strategy prevents it from reintegrating with either parental species. C. tesselatus appears to be a good candidate for an evolutionary species, “a lineage (an ancestral-descendant sequence of populations) evolving separately from others and with its own unitary evolutionary role and tendencies” (Simpson 1961, quoted in Hull

1976).

Kitcher says that all lizards in the species probably arose from a single hybrid cross but this need not be the case. Perhaps the species originated from multiple hybridization events. He also notes that if C. tesselatus were to go extinct, it could rise again from a new hybrid cross of the parent species. In such a case there would be no continuity between organisms in the species. C. tesselatus is supposed to serve as a

3 Parthenogenesis occurs when an ovum develops into a viable organism without being fertilized by a sperm. Parthenogenesis is a unisexual form of reproduction in that it requires a single parent of one sex. Parthenogenesis occurs only in females and produces only female offspring. 51 counterexample to Hull’s claim that species are continuous. If some organisms in the species are descended from hybridization event H1, while others are descended from hybridization event H2, then the species is not continuous. Some parts of C. tesselatus would find continuity with other parts of C. tesselatus only through ancestry with a lizard that is not C. tesselatus.

There are several problems with Kitcher’s account of a non-continuous species.

One might object that if two hybridization events have occurred, then there are two species, precisely because species are always continuous. Foreseeing this objection,

Kitcher responds “To hypothesize “sibling species” in this case (and in like cases) seems to me not only to multiply species beyond necessity but also to obfuscate all the biological similarities that matter” (Kitcher 184, 315). It’s unlikely that Hull would be convinced by Kitcher’s argument. Species are “made up of spatiotemporally organized parts. These parts in turn need not be and frequently are not similar to each other” (Hull

1976, 177). Morphological similarity counts for nothing on Hull’s account. No matter how similar two organisms are, they are not the same species unless they are parts of a continuous chunk of genealogical nexus. “The same species can no more re-evolve than the same organism can be born again“(Hull 1976, 184).

Another problem with the example of C. tesselatus, a problem Kitcher readily admits, is that all lizards in the species probably arose from a single hybrid cross. A second hybrid cross resulting in a second line of descent is purely hypothetical. Kitcher offers a hypothetical counterexample rather than an actual counter example, and 52 hypothetical examples are always less convincing. While past hybridization resulting in speciation is a good indicator that future hybridization resulting in speciation is possible, there is no reason to suspect that organisms resulting from the second hybridization event would be indistinguishable from organisms in the original hybrid line. Though the parents of each line would be members of the same parental species, the parents would be different lizards with their own unique genetic makeup. Given the genetic variation in both bisexual parental species and the lack of variation in the parthenogenetic hybrid line, it seems unlikely that a second hybrid would exactly resemble the first, either genetically or morphologically. If a second hybrid cross produced a second parthenogenetic line, it is likely that animals in the second line would be distinguishable from those in the first. The first line would not only be spatiotemporally individuated from the second, the animals in each line would also be genetically and morphologically distinct. There would be no reason to consider them the same species.

A third problem with Kitcher’s use of C. tesselatus as a counterexample to species continuity is that many would consider C. tesselatus not to be a species at all.

Unisexual organisms produce the same perpetually branching lines of descent that asexual organisms do. Absent are the crisscrossing genealogies that bind sexual species into single units of evolution. C. tesselatus is not a species under Mayr’s biological species concept because the lizards do not breed, and Ghiselin would not recognize it as a species because the parthenogenetic lizards do not compete for mates. Hull might consider C. tesselatus to be a species, but it’s unclear what the cohesive factor would 53 be. A line of all female parthenogenetic animals descended from a hybrid monster might fall outside the scope of Hull’s claim when he says that species are individuals.

Conclusion

Kitcher argues that species are sets and that species need not be continuous, but neither argument against Hull’s thesis is convincing. Species cannot be sets, because membership in a species waxes and wanes while set membership may not change.

Kitcher’s counterexample to species continuity fails because the group of lizards he cites might not be a species at all, is probably continuous even if it is a species, and because a second hybrid cross would be unlikely to produce animals exactly resembling those in the first hybrid line. Non-continuity with the first hybrid line would be only one of several reasons for considering the new line to be its own species. 54

CHAPTER 4: SPECIES AS UNITS OF CLASSIFICATION

In his article In Defense of Classification, John Dupré argues that species are the basic units of classification and that these units frequently do not coincide with units of evolution. Units of evolution are individuals, but units of classification are kinds; kinds are intentionally-defined classes. Classification is “the practical activity of assigning the vast number of organisms in the world to particular kinds” (Dupré 2001, 203). Dupré argues that units of evolution cannot serve as units of classification. “Species” denotes the basic units of classification, not the units of evolution.

The principle of priority in biological nomenclature states that the first valid publication of a taxon name is considered the correct use of the name, and once a name has been validly published that name may not be used again to denote some other taxon (McNeill et al., 2006). Dupré extends the principle of priority in nomenclature to the term “species”. “Here, if anywhere in biology, it seems to me, we should honor conventions of priority” (Dupré 2001, 204). “Species” is the name given to the species category, and the question is whether the species category contains units of classification or units of evolution. Species were considered the basal units of classification thousands of years before there was a theory of evolution. When the term

‘species’ was coined it referred to a class containing the basic units of classification. The name “species” is already taken, so a class containing the units of evolution may not validly be called “species”. Hull’s claim that species are individuals is predicated on the 55 notion of species as units of evolution, but his usage of “species” is invalid. If species are units of classification, then species are classes, not individuals.

Drupre’s line of reasoning mirrors Hull’s argument that proper names rigidly designate. “Species” is a common name that refers to multiple particular species such as

Gorrilla gorilla and Cygnus olor, but it is also the proper name of the species category.

The proper name “Species” rigidly designates the species category, and that category has historically been conceived of as a class of classes. Dupré also points out that for the vast majority of people who do not work in theoretical systematics species remains a classificatory concept.

Hull advocates that we consider species to be units of evolution, and thus individuals, because doing so will make evolutionary theory more coherent. Dupré points out that few working biologists are concerned with theoretical development, but that all make use of the species concept to classify the organisms they work with.

“Classification has a life of its own. Biologists in areas only tangentially connected to evolutionary theory, such as ecologists, ethnobotanists, or ethologists, need to classify organisms, as do foresters, conservationists, game keepers, and herbalists… birdwatchers, wildflower enthusiasts, or just biologically engaged members of the public, may choose to classify organisms, even if they do not need to do so. These diverse groups of people require workable classifications that allow them to communicate among themselves and to members of other such groups, record information about natural history and so on”( Dupré 2001, 204).

If units of evolution can serve as the primary units of classification, then it would be reasonable to accept Hull’s claim that species are individuals. Dupré accepts Hull’s argument that units of evolution are individuals, but for many areas of biology units of 56 evolution are not suitable candidates for the basic units of classification. In some cases all the members in a species might also be parts of a single unit of evolution. When this occurs, the set of organisms that are members of the species is identical with the set of organisms that are parts of a unit of evolution. Dupré acknowledges that this is often the case, but the species and the unit of evolution are still logically distinct. The set of parts and the set of members might be the same set, but the individual and the class are not identical.

“If, as I have acknowledged, the units of evolution are individuals, and since it seems no more than a matter of definition, units of classification must be kinds, it is not at all clear what such identification could mean. What is certainly possible is that one set of organisms could simultaneously constitute all and only the members of a kind, and all and only parts of an individual... But putting matters this way should also make it clear that once we distinguish these two kinds of units, and recognize that they belong to ontologically distinct categories, it would be strange to insist that such coincidence must always occur” (Dupré 2001, 205).

Kinds are intensionally-defined classes, but individuals are spatiotemporally-localized physical objects. It is not possible that a kind be an individual. When membership in a kind coincides with the parts of a unit of evolution one might be tempted to identify the class with the individual, but the part-whole relation is distinct from the class- membership relation. The distinction might seem trivial if membership in a species always coincides with being part of a particular unit of evolution. If this were the case, then units of evolution could also paradoxically serve as units of classification, but, as

Dupré argues, “units of evolution are often thoroughly ill-suited for this additional task”

(Dupré 2001, 205). 57

Units of evolution can be ill-suited as surrogates for units of classification in three different ways. Units of evolution can be too broad, too narrow, or simply too ambiguous to serve as practical units of classification. Dupré cites oak trees and raspberries as cases where units of evolution are too broad to serve as units of classification.

The American oaks are comprised of several dozen named species of the genus

Quercus. White oaks have leaves with smooth lobes, white heartwood, and sweet edible acorns. Red oaks have pointy tipped leaves, ruddy heartwood, and acorns infused with tannins that make them inedible. Some kinds of oak thrive in swampland, while others prefer dry rocky cliff tops. Some oaks get very large and dominate the forest, while others live as short scrubby bushes. Each kind of oak is distinguished by its unique morphology and ecological niche, despite the fact that all are inter-fertile and frequently hybridize. Morphologically and ecologically coherent lineages of oak trees are maintained despite a lack of reproductive isolation.

Units of evolution are cohesive ancestral-descendant lineages held together by sexual reproduction that share a common pool of genetic resources. On Hull’s account of species as units of evolution, all the oaks are parts of a single species, but a system of classification that countenanced a single species of oak would be unworkable for ecologists and foresters. Even squirrels can tell a sweet acorn from a bitter one. There are different kinds of oaks despite the fact that all are parts of a single unit of evolution. 58

Evolutionary theory might dictate that there is a single species of oak, but in this case

“theoretical considerations mandate a clearly suboptimal taxonomy” (Dupré 2001, 206).

Bramble berries pose a similar problem. The genus Rubus contains several hundred named species of brambles, but, according to Dupré, all are inter-fertile. I am aware of four species that grow wild in Ohio: the Black Berry, the Black Raspberry, the

Dew Berry, and the Wine Berry. Each kind of berry has its own preferred habitat, is morphologically distinct, and fruits in a different season, yet all are inter-fertile and constitute a single unit of evolution. If species are identified with units of evolution, then there is but a single species of bramble. A taxonomic system that recognized one and only one species of bramble would not reflect the biodiversity found in nature.

Sometimes units of evolution are too narrow to serve as units of classification.

Dupré notes that many species maintain morphological and ecological coherence despite geographic isolation that prevents genetic exchange. Two geographically and reproductively isolated populations form two distinct lineages. Each is a unit of evolution, but the organisms in each population are so similar that there is no practical reason to distinguish them as belonging to separate species. The problem also arises with the classification of microbes and other organisms that reproduce asexually. Each line of descent could be considered its own unit of evolution, with many lines going extinct after only a few generations. A taxonomy that used units of evolution as units of microbial classification would be terribly unwieldy even if it were operationally possible to construct, which it is not. 59

Dupré’s third objection to using units of evolution for units of classification concerns the ambiguous nature of units of evolution. It is not easy to determine where one unit of evolution begins and another ends. Dupré cites a population that becomes geographically isolated, follows its own evolutionary path, and is subsequently reintegrated with the parent population many generations later. The isolated population is subject to selection pressures particular to its environment and begins evolving independently from the parent population; it becomes a distinct unit of evolution at the moment of isolation. When the populations are reintegrated two units of evolution merge back into one. The process is similar to a channel of water that splits into two before rejoining as it flows past a river island. When the isolation lasts but a generation or two there is little reason to recognize more than one unit of evolution, but when the isolation lasts many thousands of generations evolutionary changes can accumulate making the organisms in one line morphologically distinct from those in the other. After the populations rejoin and begin interbreeding, two gene pools mix back into one and morphological traits become more or less homogenous across the entire population. Depending on the duration of isolation and the accumulation of distinguishing traits, one may choose to see a single unit of evolution or two units first splitting and then rejoining.

The history of evolution is usually depicted using a tree model where units of evolution bifurcate to form new branches but do not rejoin after splitting. In the case of isolated populations, whether or not the populations should be mapped as a bifurcating 60 line or a single straight line depends entirely on whether the populations will eventually merge back into one, or if instead they will continue evolving separately. But making such a determination would require that a theorist know what will occur in the future.

A taxonomist would need a crystal ball to determine whether two geographically isolated populations will continue evolving independently or if they will eventually merge back into one. Dupré calls the question “operationally undecidable”. Whenever it is not possible to split the genealogical nexus into distinct units of evolution, it is also undesirable to use the unit of evolution as the unit of classification. When it is not possible to determine what unit of evolution an organism is a part of, then it makes little sense to demand that we classify organisms according to their part-whole relation with a unit of evolution.

“The very idea of a unit of evolution is much vaguer than might first have been supposed. The concept of ‘reproductive isolation’ suggests a picture of evolutionary change flowing down sharply defined channels, branching at well- defined nodes—and naturally identifies units of evolution as lengths of channel terminating at nodes. A more realistic picture would be a river estuary at low tide. We find large streams of water and many side streams, some petering out, others rejoining the main channel or crossing into a different channel, and a few maintaining their integrity to the ocean; there are islands around which streams flow and rejoin; eddies and vortices; and so on. Some parts of the general flow are naturally and coherently distinguishable, and it is easy enough to recognize parts of the pattern that are definitely not parts of the same ‘unit of flow’. But in between, there are many cases where any such distinction into discrete units would be largely arbitrary” (Dupré 2001, 207).

It’s easy to conceive of species as units of evolution when the genealogical nexus is imagined as resembling the bifurcating branches of a tree, but the tree model is just an idealization. The real history of evolution is more complex. Portions of the nexus are 61 reticulate, and any attempt to carve these parts into discrete units will reflect at best an educated guess on the part of the one carving it up. Some parts of the genealogical nexus might actually resemble the bifurcating branches of a tree, but in other parts reticulate patterns dominate, and these reticulate patterns render “unit of evolution” a concept too vague to serve as the unit of classification in a workable system of taxonomy.

Conclusion

Units of evolution and units of classification are distinct logical categories. Units of evolution are individuals, but units of classification are by definition kinds. Kinds are intensionally-defined classes. Ever since Linnaeus created a standardized system of biological nomenclature, biologists have adhered to the principle of priority in nomenclature. The term “species” has historically been used to denote the basic units of classification; this is the term’s first use and hence its correct use according to the principle of priority. In order for units of evolution to serve as units of classification, the units of evolution must translate into a workable system of classification, but units of evolution yield a suboptimal taxonomy when used as the basic units of classification.

Sometimes the units of evolution are too broad to yield a workable taxonomy as in the case of oak trees and raspberries. Sometimes the units of evolution are too narrow, as in the case of microbes and other asexual organisms. And finally, reticulate evolution makes ‘unit of evolution’ too vague a concept to yield a workable taxonomy. Reticulate patterns in the genealogical nexus admit to no non-arbitrary partitioning into discrete 62 units. For these reasons units of evolution are not good candidates for units of classification.

Units of classification and units of evolution are two different kinds of things, and the term “species” properly refers to the units of classification. For millennia, the term species has rigidly designated a class populated by intensionally-defined classes. Species are classes. 63

CHAPTER 5: UNITS OF EVOLUTION AS SUBSTANCES

If species are classes, then what should we call units of evolution? Interestingly, one possible name can be found in an early work by Hull (1965). In regard to the distinction between units of identification and units of evolution, Hull quotes Sonneborn who foreshadows Dupre’s argument from priority and proposes a new name for units of evolution. “”Species” is to be reserved for the units of identification and groups of organisms which fulfill the requirements of the biological definition are to be termed

"syngens”” (Sonneborn 1957, in Hull 1965). “Syngen” was proposed as a name for units of evolution in the mid twentieth century, but maybe a much older name is applicable.

In keeping with the principle of priority, if the older name is valid then the older name should be used. I’m talking about a proper name for Hull’s ‘Species as individuals’ category, which cannot be named “species” because “species” has a prior use.

Sonneborn suggested “syngen”, but Hull rejected the neologism, attempting instead to redefine an old word to give it a new meaning, a meaning incompatible with the traditional use of the word. The much older term I propose as a proper name for Hull’s super-organismic spatiotemporally-restricted continuous individual is Secondary

Substance.

Hull makes little use of the concept ‘secondary substance’, mentioning it only as a synonym for class.

“From Socrates and Plato to Kripke and Putnam, organisms have been paradigm examples of primary substances, particulars and/or individuals, while species have served as paradigm examples of secondary substances, universals and/or classes” (Hull 1978, 338). 64

On the received view, secondary substances are universal classes, not individuals, but there is something rather odd about calling a class “substance”. Substances in general should be substantial. Individuals are substantial, but classes are non-substantial, existing only in an abstract way. They are human-invented categories that exist only in minds. R. L. Mayden notes the abstract nature of classificatory concepts. “The concept

‘category’ is a class and has no separate existence from its use in organizing objects or thoughts; categories have no reality… The category species is an artificial construct used for organization of information” (Mayden 1997, 386). If Mayden’s view is correct, and I think it is, then classes lack substance; they are not physically existing objects and therefore not substances.

Hull claims that for Socrates and Plato secondary substances are classes, but the seminal ancient treatise on substances is found in chapter five of Aristotle’s Categories.

Aristotle’s formulation of secondary substance appears to be closer to Hull’s species-as- individual than it is to species as an abstract classificatory concept.

“A substance, spoken of in the most fundamental, primary, and highest sense of the word is that which is neither said of a subject nor present in a subject; e.g., an individual man or an individual horse. Secondary substances are said to be (a) those to which, as species, belong substances which are called ‘primary’, and also (b) the genera of those species. For example, an individual man comes under the species man, and the genus of the species is animal; so both man and animal are said to be secondary substances” (Categories 5).

For Aristotle, secondary substance appears to be a biological concept. Aristotle cites only species and genera of animals as examples of secondary substances. Artifacts and minerals can also be divided into genera and species; ‘knives’ is a species of the genus 65

‘tools’ for example, but Aristotle does not include any species of non-living thing in his formulation of secondary substance. Aristotle specifically excludes universals like ‘white’ from the category of things that are secondary substances. Whiteness is a property in some substances, but whiteness itself is not a substance. There exists a class of things that are white, but this class is not a secondary substance. When Hull claims that species are individuals, he means only species of biological things. The biological nature of secondary substance in Aristotle parallels Hull’s claim that species of living things are individuals while species of non-living things such as gold are not individuals.

Aristotle says that both species and genera are secondary substances, a claim that is mirrored in Hull’s account of both species and higher taxa as individuated chunks of genealogical nexus. According to Aristotle; “Of secondary substances, the species is to a higher degree substance than a genus of it, for it is closer to primary substance than a genus of it is" (Categories 5). On Hull’s view, species are individuals to a higher degree than genera because species possess more individuating properties. Species-as- individuals are spatiotemporally-distinct, continuous, and cohesive. Higher taxa are spatiotemporally-distinct and continuous, but they lack cohesion. Higher taxa such as genera are spatiotemporally-individuated parts of the genealogical nexus, so they are individuals, but they are diffuse individuals, less substantial than species which are likewise spatiotemporally-individuated but also bound together by cohesive factors such as interbreeding. Particular organisms are parts of species and species are parts of genera on Hull’s view, but species have higher claim to the title individual than do 66 genera, because particular organisms, primary substances, are bound more tightly into species than they are into genera. Species are individuals to a higher degree than genera for Hull, and species are substances to a higher degree than genera for Aristotle.

To be a lineage is a property of both secondary substances and species as individuals. Hull says “From Darwin to the present, evolutionary theory has always included a strong principle of heredity” (Hull 1976, 180). While this is true, the principle of heredity is not original to evolutionary theory. The ancients were not surprised when an olive pit grew into an olive tree or when a horse gave birth to a horse. A strong principle of heredity has been central to biological knowledge since before biology was even a science. The principle can be found in Aristotle’s maxim that “like produces like”.

Aristotle’s account of heredity is muddied a little by spontaneous generation, but for large animals and common plants the principle is clear. The only way to be a horse is to be born of a horse, and the only way to be an olive tree is to be sprouted from an olive seed. Aristotle was familiar with the concept of genealogical lineages even though he was not aware that lineages evolve. The ancients knew that a seed is materially continuous with both the plant that sprouts it and the plant that springs from it. When considering successive generations of like producing like, each generation linked by material cause to both its progenitor and its progeny, it’s not unlikely that the ancients conceived of ancestral descendant lineages as individuals. The individuality of lineages would have been especially evident to those who cloned vines for grape production. A part cut from a vine and planted beside it seems in some sense to be very much still a 67 part of the original vine. If linages are conceived of as individuals, then it makes sense to call them substances.

Material continuity is one possible reason that Aristotle considered species of living things to be substances. Material continuity yields spatiotemporal continuity, a property that Hull cites as evidence that species are individuals. The reason for considering species to be individuals and the reason for considering species to be substances has a common origin in material continuity from generation to generation.

Material continuity neither entails nor prohibits evolution; it’s merely a property of lineages that makes them substantial individuals. Secondary substances do not evolve on Aristotle’s view, but otherwise they are very much like Hull’s species-as-individuals.

Particular organisms are parts of secondary substances just as particular organisms are parts of species-as-individuals. On Aristotle’s view, linages are timeless and unchanging while on Hull’s view they evolve, but in both cases organisms are parts of larger transgenerational physical individuals.

Aristotle didn’t know about evolution, but in a possible world where he found out about it and was forced to adapt his metaphysics to account for it, Aristotle would probably have identified units of evolution with secondary substances. Evolution in a secondary substance would seem analogous to development in a primary substance, a process his system already accounts for.

My arguments that “secondary substance” is the earliest valid synonym for

“species as individuals” are not exhaustive, but a more thorough investigation of the 68 question is beyond the scope of this project. If it turns out that secondary substance is not a valid synonym for ‘species as individual’, then “syngen” might be the appropriate name. Either way, “species as individuals” should be deprecated in favor of some other name, because “Species” has a previous valid use that prevents species from being individuals. On any account, Hull’s spatiotemporally-continuous trans-generational historical individual is a physical object, and therefore a substance. Primary substances are particular organisms, so the secondary sense of substance seems to be a good fit.

Maybe the difference between primary substance and secondary substance is 3D continuity versus 4D continuity, the same difference that occurs at the radical break in continuity between an organism and its population or kinship group. 69

CHAPTER 6: POSTSCIPT ON SPECIES AND UNITS OF EVOLUTION

Dupré agrees with Hull that for many kinds of organisms the unit of evolution is distinct enough to serve as the best character for delineating species, but even in these cases species should not be identified with the unit of evolution. The identity relation will not work; species as taxonomic categories are abstractions with no physical reality while substances are physically existing things. A physical thing cannot be identical with a non-physical thing, so a unit of evolution cannot be identical with a species.

Many organisms, including most large animals, are parts of well-defined units of evolution. When this occurs, theoretical considerations yield the optimal taxonomy.

When a species is defined theoretically, the unit of evolution is an individual, and the species is the class of all organisms that are parts of the unit of evolution. The species remains an intensionally-defined class. The condition for membership in the species is to be a part of a particular spatiotemporally-individuated, continuous, and cohesive evolving lineage, which I have argued is a substance in Aristotle’s secondary sense.

Species names such as Gorilla gorilla refer to both the transgenerational substance and the class of animals whose members are its parts. 70

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